The invention relates generally to enclosures for electronic hardware. More particularly, the invention relates to enclosures made of composite materials and that provide electromagnetic shielding for electronic hardware installed in such enclosures.
As is well known, electronic equipment and hardware is susceptible to electromagnetic interference (EMI), whether it be generated from radio waves, interference from nearby equipment, meteorological disturbances and so forth. In applications such as avionics, both military and commercial, such interference can present an unacceptable risk to proper operation of the hardware. Therefore, enclosures for sensitive hardware usually provide for EMI shielding, and in fact such shielding is often necessitated by military and commercial specifications.
Although enclosures made of metal such as aluminum provide good shielding against EMI, such materials add significantly to the weight of the equipment. Composite materials hold great promise for replacing heavier metal materials for enclosures with lightweight but comparably strong materials. Shielding with such composite materials can be accomplished by the use of conductive fibers in the composite material used to form the walls of the enclosure.
Although the general concept of using composite materials for shielded enclosures is well known, their use to date has been limited because of some associated problems. Although composite materials can exhibit excellent structural characteristics, in order to achieve sufficient strength, stiffness and vibrational requirements a trade off is made as to weight. In other words, the weight savings is not necessarily as great as expected due to the need for additional material in order to meet structural integrity requirements. Composite materials that meet the structural requirements also tend to be very expensive, thus substantially increasing the cost of enclosures made of such materials beyond the cost of conventional metal enclosures.
Most enclosures for electronics hardware include a card guide and heat exchanger mounted therein. The card guide, of course, is used to hold one or more electronics modules, and typically includes a plurality of guide ribs that support the modules, and a cold wall that the ribs are attached to. An electronic module typically includes one or more printed circuit boards mounted on a thermally conductive substrate (thermal plane) such as a flat metal sheet or plate. The module is held in the card guide by a suitable clamping device that clamps or wedges the module against one or more of the ribs.
Because a shielded enclosure is tightly sealed from the external environment, heat dissipated from the electronic components must be extracted from the enclosure. Typically, this is accomplished by the card guide, which is metal, and the heat exchanger. Heat is transferred from the circuit boards, through the thermal plane and clamping device to the card guide. The heat exchanger is usually in good thermal contact with the card guide and draws the heat therefrom and transfers it to actively cooled heat transfer fins, and thereafter to the outside region of the enclosure.
Although metal card guides provide good thermal dissipation, their thermal characteristics are not expected to be adequate for future specifications that will require higher heat dissipation in ever smaller enclosures. Electronics manufacturers continually improve their ability to miniaturize and densely pack electronic components into smaller envelopes, thus constantly pushing the performance requirements for dissipating heat from the enclosures. Metal card guides and enclosures will likely not meet these requirements in the future, and in any event will always have an undesirable weight factor, even when the basic enclosure walls are made of composite material.
The need exists, therefore, for a shielded enclosure that exhibits as good as or better structural performance as a conventional enclosure, and that exhibits substantially improved thermal transfer characteristics, while at the same time maintaining weight and cost advantages over known shielded enclosures.
U.S. Statutory Invention Record H526 relates to non-metallic chassis structure with electromagnetic field attenuating capability. This record refers to an electronic equipment chassis structure using non-metallic plastic material such as epoxy-impregnated fiberglass together with layers of shielding metal film, preferably of copper overlaid with nickel.